A pressure-measuring plug for a combustion engine is known from EP2444786A1. The pressure-measuring plug comprises a plug body, a ring shaped sensing structure and a circular membrane. The plug body comprises an external thread section for mounting the plug body into a cylinder head of the combustion engine and a body tip section facing in use the combustion chamber. The ring-shaped sensing structure comprises an outer section, an inner section and an annular diaphragm. The plug body is attached to the outer section. The ring-shaped sensing structure allows the inner section to move relatively to the outer section along a cylinder axis of the ring-shaped sensing structure by deformation of the diaphragm. Strain gauges attached to the sensing structure sense the deformation of the sensing structure. The deformation has a relationship with the pressure acting on the pressure-measuring plug.
The circular membrane comprises an outer part coupled to the outer section and an inner part coupled to the inner section. The membrane provides a sealing protecting the annular diaphragm against the harsh environment in a combustion chamber.
The circular membrane and the body tip section form a chamber. A passage of the body tip section provides an open connection between the plug chamber and the combustion chamber. In this way, the pressure in the combustion chamber could act on the circular membrane and the inner section of the ring-shaped sensing structure.
In the future, advanced combustion strategies for diesel and Otto engines depend on the existence of accurate pressure feedback from each combustion cylinder during the entire engine cycle (compression—combustion—exhaust cycle). These strategies may or may not include Homogeneous Charge Compression Ignition (HCCI) combustion, and can result in high pressure release rates which require fast and accurate pressure response.
Accuracy of the sensor signal over the life time of the sensor is vital for correct closed loop combustion strategies. Drift in the sensor signal is known to occur by soot accumulation on the circular membrane. Accumulated soot reduces sensitivity to pressure changes and consequently to a loss of signal. A layer of soot changes the mechanical characteristics of the circular membrane and reduces the force transfer via the inner section to the diaphragm of the sensing structure where the force measuring elements in the form of strain gauges, are located. Soot can permanently change the sensor characteristics and is therefore an important durability and stability factor for the control of the engine.
During the combustion process, soot which could be any of elemental carbon, unburned fuel, sulfates etc., has been observed to condense/deposit on various engine components including the pressure-measuring plug interface directly exposed to combustion gasses. Soot is transported by the combustion gasses during and after the combustion process. Every surface or device in contact with combustion gasses is potentially affected by soot accumulation and, depending on its function, the device function can change over the life time of the engine when accumulation of soot is changing its characteristics.
Due to the nature of the pressure-measuring sensor in contact with the cooled engine head which creates a large temperature delta between hot combustion gasses and “cold” pressure-measuring sensor, combustion condensates collect on sensor surfaces (thermophoretic/diffusion-phoretic mechanisms) which also can adversely affect the sensor performance over time.